1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly, to a method for forming a dielectric film of a capacitor.
2. Description of the Related Art
With the rapid increase in the integration level of semiconductor devices, the area of a unit memory cell has been reduced. Accordingly, it is necessary to form a capacitor having a sufficient cell capacitance for device operation.
The capacitance of a semiconductor memory device is an important factor which determines a memory capacity of the memory device. Reduced capacitance lowers the reading capability of a memory cell and increases a soft error rate. Therefore, in order to attain a highly integrated semiconductor memory device, a cell capacitance greater than a predetermined value must be obtained.
Various methods for increasing the capacitance within a limited cell area have been proposed. Among them, a typical method is to increase an effective area of a capacitor by increasing the electrode surface area by growing hemispherical grains (HSGs) to obtain a rugged surface of a lower electrode.
Generally, the capacitance of a capacitor is inversely proportionate to the thickness of a dielectric film and is directly proportionate to the contact area of the dielectric film with an electrode. Thus, it is necessary to minimize the thickness of the dielectric film and to maximize the contact area of the dielectric film with the electrode. To this end, when the dielectric film is formed on a lower electrode, it is important to obtain excellent step coverage.
Particularly, when the dielectric film is formed on the rugged surface of the lower electrode, in order to suppress leakage current while obtaining large capacitance, it is imperative to obtain excellent step coverage.
However, if a dielectric film is formed using a conventional chemical vapor deposition (CVD) or physical vapor deposition (PVD) method, it is difficult to gain excellent step coverage. Particularly, in the conventional CVD method, although a dielectric film having relatively good step coverage can be obtained by a deposition process utilizing a surface kinetic mode, reactants are simultaneously deposited onto the substrate. Thus, it is difficult to adjust step coverage in a specific portion as needed.
In particular, in a three-dimensional lower electrode having a rugged surface, excellent step coverage is difficult to obtain since the film deposited on a concavity formed by the rugged surface tends to be thin.
In a capacitor comprised of the lower electrode having a rugged surface, a greater electric potential acts upon an electrode portion having a smaller radius of curvature than upon a portion having a larger radius of curvature.
Generally, in a spherical capacitor formed in the capacitor having a rugged surface of the lower electrode, the electric potential is proportionate to the difference between radii of curvatures of two electrodes forming the capacitor, and is inversely proportionate to the product of the respective radii of curvatures. On the other hand, the capacitance is inversely proportionate to the difference between radii of curvatures of two electrodes forming the capacitor, and is regularly proportionate to the product of the respective radii of curvatures. Here, whereas the capacitance is affected by the overall area of the capacitor, the leakage current characteristic represented by the electric potential is determined by the weakest portion, i.e., an electrode surface portion having the smallest radius of curvature formed by the rugged surface. In other words, in the capacitor having a rugged surface of the lower electrode, a greater electric potential acts upon the portion having the smaller radius of curvature on the electrode surface than upon the portion having the larger radius of curvature, and thus more leakage current flows in the portion having the smaller radius of curvature than in the portion having the larger radius of curvature.
In the above description, the portions having the smaller radii of curvatures of the capacitor electrode means a plurality of concavities on the rugged surface of the lower electrode. These concavities are liable to cause a dielectric film to be formed thereon thinly, compared to convexities formed by the rugged surface when the dielectric film is formed by a conventional deposition method. When the dielectric film is to be formed, at a concavity having a small radius of curvature, thickly enough to prevent leakage current from increasing at a thin dielectric film portion due to the concentrated electric potential, a conventional method for forming a dielectric film unnecessarily increases the thickness at other dielectric film portions, thereby failing to obtain the desired capacitance.
As a result, according to the conventional method for forming a dielectric film, excellent step coverage is difficult to obtain. Although excellent step coverage could be obtained, increased leakage current at a concavity having a small radius of curvature cannot be overcome.